Decolorization of water-insoluble Sudan dyes was studied with Shewanella oneidensis MR-1, which removed 66.8%, 43.4%, 56.0% and 33.7% Sudan I-IV in 104 h, respectively and reduced Sudan I to aniline and 1-amino-2-naphthol. Lactate was identified as the most efficient electron donor for Sudan I reduction. Improved reduction performance was obtained in the presence of higher lactate or biomass concentration. The correlation between specific reduction rate and initial Sudan I concentration could be described with Michaelis-Menten kinetics (V(max)=1.8 mg Sudan I mg cell(-1) h(-1) and K(m)=5.3 mg l(-1)). The addition of anthraquinone-2-sulfonate stimulated the reduction significantly whereas the presence of 2-hydroxy-1,4-naphthoquinone had little enhancing effect. The main azoreductase activity was found with membrane-bound proteins of MR-1 and no reduction occurred when Sudan I was incubated with cell extracts. These data indicated for the first time that Shewanella could reduce solid-phase Sudan dye particles.